[Bug] mcp Unrecognized field \"tools\

[cookiejoo]

i used cursor add mcp，

There is indeed a 'tools' field under 'capabilities'.
However, according to the MCP specification on Capabilities negociation , the client should not have a 'tools' field as 'capabilities', since it is up to the server to set them.

Steps to Reproduce
bom version:1.0.0-M6

Run the example Spring AI MCP WebFlux server here: https://github.com/spring-projects/spring-ai-examples/tree/main/model-context-protocol/weather/starter-webflux-server
Launch it
Add the server url under mcpServers (type: sse)
Launch the Cursor MCP Server
Log :
event:endpoint
data:/mcp/message

event:message
data:{"jsonrpc":"2.0","id":0,"error":{"code":-32603,"message":"Unrecognized field "tools" (class io.modelcontextprotocol.spec.McpSchema$ClientCapabilities), not marked as ignorable (3 known properties: "experimental", "roots", "sampling"])\n at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.modelcontextprotocol.spec.McpSchema$InitializeRequest["capabilities"]->io.modelcontextprotocol.spec.McpSchema$ClientCapabilities["tools"])"}}

[deemkeen]

https://witsyai.com/ has the same problem, defining the tools field in the client capabilities, it seems that many mcp clients don't implement the spec correctly

[tzolov]

Thanks for bringing this up @cookiejoo, @deemkeen.

However the McpSchema.ClientCapabilities class is compliant with the MCP spec and doesn't include a "tools" field. The issue appears to be that Cursor is sending non-compliant JSON with extra fields not in the specification.

The error:

data:{"jsonrpc":"2.0","id":0,"error":{"code":-32603,"message":"Unrecognized field "tools" (class io.modelcontextprotocol.spec.McpSchema$ClientCapabilities), not marked as ignorable (3 known properties: "experimental", "roots", "sampling"])\n at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.modelcontextprotocol.spec.McpSchema$InitializeRequest["capabilities"]->io.modelcontextprotocol.spec.McpSchema$ClientCapabilities["tools"])"}}

occurs because Cursor is sending a tools field in the capabilities object that is not compliant with the spec and the receiving system doesn't recognize.

I suggest:

• Report this spec incompatibility issue to Cursor and Witsyai
• As a workaround, we can relax the client-side JSON schema strictness in the MCP Java SDK to ignore unknown properties

The core issue is with Cursor's client implementation, not our client-side code.

[cookiejoo]

感谢您提出这个问题@cookiejoo，@deemkeen。

但是McpSchema.ClientCapabilities类符合 MCP 规范，不包含“工具”字段。问题似乎是 Cursor 发送了不符合规范的 JSON，其中包含规范中没有的额外字段。

错误：

data:{"jsonrpc":"2.0","id":0,"error":{"code":-32603,"message":"Unrecognized field "tools" (class io.modelcontextprotocol.spec.McpSchema$ClientCapabilities), not marked as ignorable (3 known properties: "experimental", "roots", "sampling"])\n at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.modelcontextprotocol.spec.McpSchema$InitializeRequest["capabilities"]->io.modelcontextprotocol.spec.McpSchema$ClientCapabilities["tools"])"}}

发生这种情况的原因是 Cursor 发送的tools功能对象中有一个字段不符合规范，并且接收系统无法识别。

我建议：

• 将此规范不兼容问题报告给 Cursor 和 Witsyai
• 作为一种解决方法，我们可以放宽MCP Java SDK中客户端 JSON 模式的严格性，以忽略未知属性

核心问题在于 Cursor 的客户端实现，而不是我们的客户端代码。

Thank you, I will report this issue to the cursor